Apparatus for converting direct current to alternating current

ABSTRACT

An inverter for converting an input direct current (DC) waveform from a DC source to an output alternating current (AC) waveform for delivery to an AC grid includes an input converter, an output converter, and an active filter, each of which is electrically coupled to a bus. The bus may be a DC bus or an AC bus. The input converter is configured to convert the input DC waveform to a DC or AC bus waveform. The output converter is configured to convert the bus waveform to the output AC waveform at a grid frequency. The active filter is configured to reduce a double-frequency ripple power of the bus waveform by supplying power to and absorbing power from the power bus.

CROSS-REFERENCE TO RELATED U.S. PATENT APPLICATION

This application is a continuation application of U.S. application Ser.No. 12/563,499, now U.S. Pat. No. 8,279,642, entitled “Apparatus forConverting Direct Current to Alternating Current using an Active Filterto Reduce Double-Frequency Ripple Power of Bus Waveform,” which wasfiled on Sep. 21, 2009 and which claims priority under 35 U.S.C. §119(e)to U.S. Provisional Patent Application Ser. No. 61/230,546 entitled“Apparatus for Converting Direct Current to Alternating Current,” byRobert S. Balog, Jr. et al., which was filed on Jul. 31, 2009, theentirety of both of which is hereby incorporated by reference.

Cross-reference is made to U.S. Utility patent application Ser. No.12/563,499 entitled “Apparatus for Converting Direct Current toAlternating Current” by Patrick P. Chapman et al., which was filed onSep. 21, 2009, and to U.S. Utility patent application Ser. No.11/871,015 entitled “Methods for Minimizing Double-Frequency RipplePower in Single-Phase Power Conditioners” by Philip T. Krein. et al.,which was filed on Oct. 11, 2007, the entirety of both of which isincorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates, generally, to power converters forconverting direct current (DC) power to alternating current (AC) powerand, more particularly, to apparatuses and methods for converting DCpower to AC power suitable for supplying energy to a an AC grid or ACload coupled to the AC grid.

BACKGROUND

Power inverters convert a DC power to an AC power. For example, somepower inverters are configured to convert the DC power to an AC powersuitable for supplying energy to an AC grid and, in some cases, an ACload coupled to the AC grid. One particular application for such powerinverters is the conversion of DC power generated by an alternativeenergy source, such as photovoltaic cells (“PV cells” or “solar cells”),fuel cells, DC wind turbine, DC water turbine, and other DC powersources, to a single-phase AC power for delivery to the AC grid at thegrid frequency.

A basic electrical property of a single-phase AC power system is thatthe energy flow includes both an average power portion that deliversuseful energy from the energy source to the load and a double-frequencyportion that flows back and forth between the load and the source:

p(t)=Po+Po*cos(2ωt+φ)  Equation 1

In applications involving inverters, the double-frequency portionrepresents undesirable ripple power that, if reflected back into the DCpower source in a significant amount, may compromise performance of thesource. Such concerns are particularly relevant to photovoltaic cellswherein the power delivered by each photovoltaic cell may vary inmagnitude over time due to temporal variations in operating conditionsincluding changes in sunlight intensity, angle of incidence of sunlight,ambient temperature and other factors. As such, photovoltaic cells havean operating point at which the values of the current and voltage of thecell result in an ideal or “maximum” power output. This “maximum powerpoint” (“MPP”) is a function of environmental variables, including lightintensity and temperature. Inverters for photovoltaic systems mayinclude some form of maximum power point tracking (“MPPT”) as amechanism of identifying and tracking the maximum power point (“MPP”)and adjusting the inverter to exploit the full power capacity of thecell at the MPP. Extracting maximum power from a photovoltaic cellrequires that the cell operate continuously at its MPP. As such,fluctuations in power demand, caused, for example, by thedouble-frequency ripple power being reflected back into the cell, maycompromise the ability of the inverter to deliver the cell's maximumpower.

In typical inverters, the double-frequency ripple power is managed bystoring and delivering energy at twice the AC frequency. To do so, apassive or active filter is typically used to manage thedouble-frequency ripple power on the input side of the inverter. Inpassive filtering arrangements, a large capacitance is typicallyrequired because the capacitive device must support the DC bus voltagewithout imposing significant voltage ripple on the DC bus. However, inactive filter arrangements, a relatively smaller capacitance may be usedbecause the capacitive device need not support the DC bus voltage.Because the active filter “isolates” the internal capacitor from the DCbus, the voltage variation across the internal capacitor can berelatively large and the value of the capacitor may be made relativelysmall.

In a typical photovoltaic power system, an inverter may be associatedwith one or more solar cell panel. For example, some systems includestrings of solar cell panels that deliver a relatively high, combinedvoltage (e.g., nominal 450 V) to a single, large inverter.Alternatively, in other systems such as a distributed photovoltaic powersystem, an inverter may be associated with each solar cell panel. Insuch systems, the solar cell panels are typically small, relatively lowvoltage (e.g., 25 V). The inverter may be placed in close proximity tothe associated solar cell panel to increase the conversion efficiency ofthe overall system.

SUMMARY

According to on aspect, an inverter for converting an input directcurrent (DC) waveform from a DC source to an output alternating current(AC) waveform for delivery to an AC grid may include an input converter,an output converter, an active filter. The input converter may beelectrically coupled to a power bus and may include a transformer. Theinput converter may be configured to convert the input DC waveform to abus waveform supplied to the power bus. The output converter may beelectrically coupled to the power bus and configured to convert the buswaveform to the output AC waveform at a grid frequency. The activefilter may be electrically coupled to the power bus. Additionally, theactive filter may be configured to reduce a double-frequency ripplepower of the bus waveform by supplying power to and absorbing power fromthe power bus.

In some embodiments, the power bus may be embodied as a DC bus and thebus waveform may be embodied as a first DC waveform. In suchembodiments, the input converter may include a first inverter circuitelectrically coupled to the DC source and a primary winding of thetransformer. The first inverter circuit may be configured to convert theinput DC waveform to a first AC waveform at a first frequency.Additionally, the transformer may be configured to converter the firstAC waveform to a second AC waveform at the first frequency. The inputconverter may also include a rectifier circuit electrically coupled to asecondary winding of the transformer and the power bus. The rectifiercircuit may be configured to rectify the second AC waveform to producethe first DC waveform. The input converter may also include an isolatedboost converter electrically connected to the DC source and the firstinverter circuit.

In some embodiments, the output converter may include a second invertercircuit electrically coupled to the power bus. The second invertercircuit may be configured to convert the first DC waveform to the outputAC waveform. The output converter may also include a filter circuitelectrically coupled to the second inverter circuit. The filter circuitmay be configured to filter the output AC waveform.

In some embodiments, the active filter may include a bridge circuitelectrically coupled to an energy storage device. The bridge circuit maybe a half-bridge or full-bridge circuit and the energy storage devicemay be embodied as an inductor-capacitor (LC) circuit. The active filteris may be configured to maintain a substantially constant voltage acrossthe second inverter circuit. In some embodiments, the output AC waveformmay include an average component and a time-varying component. In suchembodiments, the active filter may be configured to supply substantiallyall of the time-varying component of the output AC waveform.Additionally, the time-varying component may include a component havinga frequency substantially equal to twice the grid frequency.

The active filter may include a third inverter circuit and an energystorage device in some embodiments. The third inverter circuit may beconfigured to control the time-varying component of the output ACwaveform by controlling a time-varying current of the energy storagedevice. In some embodiments, the third inverter circuit is configured togenerate a third AC waveform at the energy storage device. The third ACwaveform may have a frequency substantially equal to a harmonicfrequency of the grid frequency. Additionally, the third AC waveform maybe shifted by π/4 radians relative to a phase of the output AC waveform.In some embodiments, the third inverter circuit may be embodied as afull bridge circuit.

In some embodiments, the transformer may include a tertiary winding. Insuch embodiments, the active filter may be connected to the tertiarywinding. In such embodiments, the active filter may include a rectifiercircuit connected to the tertiary winding, an inverter circuit connectedto the rectifier circuit, and an energy storage device. In someembodiments, the power bus may be embodied as a first DC bus. In suchembodiments, the inverter further includes a second DC bus electricallycoupled to the tertiary winding. In other embodiments, the power bus maybe embodied as a first AC bus. In such embodiments, the inverter mayfurther include a second AC bus electrically coupled to the tertiarywinding. Additionally, the active filter may be connected to the secondAC bus and include an energy storage device connected to a center tap ofthe tertiary winding.

In embodiments in which the power bus is an AC bus, the input convertermay be embodied as a one of a voltage-sourced full bridge converter anda voltage-sourced push-pull converter. Alternatively, the inputconverter is one of a current-sourced full bridge converter and acurrent-sourced push-pull converter. In some embodiments, the buswaveform is embodied as a first AC waveform having a first frequency. Insuch embodiments, the output converter may include a frequency convertercircuit configured to convert the second AC waveform to a third ACwaveform at the grid frequency.

In some embodiments, the inverter may further include a controllercircuit. The controller circuit may be electrically coupled to the inputconverter and configured to control the input converter to generate thebus waveform based on a maximum power point tracking of the DC source.Additionally or alternatively, the controller circuit may beelectrically coupled to the output converter and configured to controlthe output converter to generate the output AC waveform using pulsewidth modulation. Additionally or alternatively, the controller circuitmay be electrically coupled to the active filter and configured tocontrol the active filter to supply a time-varying waveform to the powerbus to reduce the double-frequency ripple power. Additionally, in someembodiments, the DC source may be embodied as one of a photovoltaic celland a fuel cell.

According to another aspect, an apparatus includes a solar panel and aninverter. The solar panel may include a solar cell configured togenerate a first direct current (DC) waveform in response to receivingan amount of sunlight. The inverter may be coupled to the solar cellpanel. The inverter may be configured to receive the first DC waveformand convert the first DC waveform to an output alternating current (AC)waveform. In some embodiments, the inverter may include an inputconverter, and output converter, and an active filter. The inputconverter may be electrically coupled the solar cell and a DC bus andmay be configured to convert the first DC waveform to a second DCwaveform supplied to the DC bus. The output converter may beelectrically coupled to the DC bus and may be configured to convert thesecond DC waveform to the output AC waveform at a first frequency. Theactive filter may be electrically coupled to the DC bus and may beconfigured to reduce a double-frequency ripple power of the second DCwaveform by supplying power to and absorbing power from DC power bus.

In some embodiments, the input converter may include a first invertercircuit and a transformer. The first inverter circuit may be beingelectrically coupled to the solar cell and a primary winding of thetransformer and may be configured to convert the first DC waveform to afirst AC waveform at a first frequency. The transformer may beconfigured to convert the first AC waveform to a second AC waveform atthe first frequency. The input converter may further comprise arectifier circuit electrically coupled to a secondary winding of thetransformer and the DC bus. The rectifier circuit may be configured torectify the second AC waveform to produce the second DC waveform.

The output converter may include a second inverter circuit electricallycoupled to the DC bus and configured to convert the second DC waveformto the output AC waveform. The active filter may include an energystorage device and a third inverter circuit configured to generate athird AC waveform at the energy storage device. The third AC waveformmay have a frequency substantially equal to a harmonic of the gridfrequency. Additionally, in some embodiments, the third AC waveform maybe shifted in phase relative to the grid frequency. For example, the ACwaveform may be shifted by π/4 radians relative to a phase of the outputAC waveform.

In some embodiments, the inverter may include a controller circuit. Thecontroller circuit may be electrically coupled to the input converterand configured to control the input converter to generate the second DCwaveform based on a maximum power point tracking of the solar cell.Additionally or alternatively, the controller circuit may beelectrically coupled to the output converter and configured to controlthe output converter to generate the output AC waveform using pulsewidth modulation. Additionally or alternatively, the controller circuitmay be electrically coupled to the active filter and configured tocontrol the active filter to supply a time-varying waveform to the DCbus to reduce the double-frequency ripple power.

According to a further aspect, a power inverter may include a directcurrent (DC) bus, an input converter electrically coupled to the DC bus,an output converter electrically coupled to the DC bus, and an activefilter electrically coupled to the DC bus. The input converter mayinclude (i) a transformer having a primary winding and a secondarywinding, (ii) a first inverter circuit electrically coupled to theprimary winding and configured to convert an input DC waveform to afirst alternating current (AC) waveform at the primary winding, thetransform being configured to convert the first AC waveform to a secondAC waveform, and (iii) a rectifier circuit electrically coupled to thesecondary winding and the DC bus. The rectifier circuit may beconfigured to rectify the second AC waveform to produce a second DCwaveform on the DC bus. The output converter may include a secondinverter circuit electrically coupled to the DC bus and configured toconvert the second DC waveform to an output AC waveform suitable fordelivery to an AC grid. The active filter may be configured to reducedouble-frequency ripple power of the second DC waveform by supplyingpower to and absorbing power from DC power bus.

In some embodiments, the active filter may include an energy storagedevice and a third inverter circuit. The third inverter circuit may beconfigured to generate a third AC waveform at the energy storage device.The third AC waveform may have a frequency substantially equal to aharmonic of the grid frequency and, in some embodiments, may be shiftedby a phase amount (e.g., π/4 radians) relative to a phase of the outputAC waveform. Additionally, in some embodiments, the inverter may furtherinclude a controller circuit. The controller circuit may be electricallycoupled to the input converter and configured to control the inputconverter to generate the second DC waveform based on a maximum powerpoint tracking of the solar cell. Additionally or alternatively, thecontroller circuit may be electrically coupled to the output converterand configured to control the output converter to generate the output ACwaveform using pulse width modulation.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of one embodiment a system forconverting DC power to AC power;

FIG. 2 is a simplified block diagram one embodiment of an ACphotovoltaic module of the system of FIG. 1;

FIG. 3 is a simplified block diagram of one embodiment of an inverter ofthe system of FIG. 1;

FIG. 4 is a simplified block diagram of one embodiment of an inputconverter of the inverter of FIG. 3;

FIG. 5 is a schematic of one embodiment of the input converter of FIG.4;

FIG. 6 is a simplified block diagram of one embodiment of the outputconverter and active filter of the inverter of FIG. 3;

FIG. 7 is a schematic of one embodiment of the output converter andactive filter of FIG. 6;

FIG. 8 is a schematic of one embodiment of an EMI filter of the outputconverter of FIG. 6;

FIG. 9 is a schematic of another embodiment of the active filter of FIG.6;

FIG. 10 is a schematic of another embodiment of the active filter ofFIG. 6;

FIG. 11 is a schematic of another embodiment of the active filter ofFIG. 6;

FIG. 12 is a simplified block diagram of another embodiment of the inputconverter of FIG. 4;

FIGS. 13A-13D are schematics of various embodiments of the inputconverter of FIG. 12;

FIG. 14 is a simplified block diagram of another embodiment of theoutput converter and active filter of FIG. 3;

FIGS. 15A-15B are schematics of various embodiments of the outputconverter of FIG. 14;

FIG. 16 is a schematic of one embodiment an inverter including the inputinverter of FIG. 12 and the output converter and active filter of FIG.14; and

FIG. 17 is a schematic of one embodiment an inverter including the inputinverter of FIG. 12 and the output converter and active filter of FIG.14.

DETAILED DESCRIPTION

While the concepts of the present disclosure are susceptible to variousmodifications and alternative forms, specific exemplary embodimentsthereof have been shown by way of example in the drawings and willherein be described in detail. It should be understood, however, thatthere is no intent to limit the concepts of the present disclosure tothe particular forms disclosed, but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

References in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to effect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

Some embodiments of the disclosure, or portions thereof, may beimplemented in hardware, firmware, software, or any combination thereof.Embodiments of the disclosure may also be implemented as instructionsstored on a tangible, machine-readable medium, which may be read andexecuted by one or more processors. A machine-readable medium mayinclude any mechanism for storing or transmitting information in a formreadable by a machine (e.g., a computing device). For example, amachine-readable medium may include read only memory (ROM); randomaccess memory (RAM); magnetic disk storage media; optical storage media;flash memory devices; and others.

Referring to FIG. 1, a system 100 for supplying alternating current(hereinafter “AC”) power to an AC grid 102 at a grid frequency includesa direct current (hereinafter “DC”) source 104 and an inverter 106. TheDC source 104 may be embodied as any type of DC source configured togenerate or produce a DC power, which is supplied to the inverter 106.For example, the DC power may be embodied as a photovoltaic solar cellor array, a fuel cell, a wind turbine configured to generate a DC power(e.g., via a rectifying circuit), a water turbine configured to generatea DC power, or other unipolar power source.

The inverter 106 is electrically connected to the DC source 104 andconfigured to convert a DC waveform generated by the DC source 104 to anAC waveform suitable for delivery to the AC grid 102 and, in someembodiments, loads coupled to the AC grid 102. The AC grid may beembodied as, for example, a utility power grid that supplies utility ACpower to residential and commercial users. Such utility power grids maybe characterized as having an essentially sinusoidal bipolar voltage ata fixed grid frequency (e.g., f=ω/2π=50 Hz or 60 Hz).

The inverter 106 includes a plurality of circuits to facilitate theconversion of the DC power to the AC power as discussed in more detailbelow. In some embodiments, the inverter 106 may include one or moreprocessing circuits 108 and one or more memory circuits 110. Theprocessing circuit 108 may be embodied as any type of processor andassociated circuitry configured to perform one or more of the functionsdescribed herein. For example, the processing circuit 108 may beembodied as or otherwise include a single or multi-core processor, anapplication specific integrated circuit, a collection of logic devices,or other circuits. The memory circuits 110 may be embodied as read-onlymemory devices and/or random access memory devices. For example, thememory circuit 110 may be embodied as or otherwise include dynamicrandom access memory devices (DRAM), synchronous dynamic random accessmemory devices (SDRAM), double-data rate dynamic random access memorydevices (DDR SDRAM), and/or other volatile or non-volatile memorydevices. The memory circuits 108 may have stored therein a plurality ofinstructions for execution by the processing circuits to controlparticular functions of the inverter as discussed in more detail below.

As discussed above, in some embodiments, the DC source 104 may beembodied as one or more photovoltaic cells. In such embodiments, the DCsource 104 and the inverter 106 may be associated with each other toembodied an AC photovoltaic module (ACPV) 112 as illustrated in FIG. 2.The ACPV 112 includes a DC photovoltaic module (DCPV) 114, whichoperates as the DC source 104, electrically coupled to the inverter 106.The DCPV 114 includes one or more photovoltaic cells and is configuredto deliver a DC waveform to the inverter 106 in response to receiving anamount of sunlight. The DC power delivered by the ACPV 112 is a functionof environmental variables, such as, e.g., sunlight intensity, sunlightangle of incidence and temperature. In some embodiments, the inverter106 is positioned in a housing 116 of the ACPV 112. Alternatively, theinverter 106 may include its own housing 118 secured to the housing 116of the ACPV 112. Additionally, in some embodiments, the inverter 106 isseparate from the housing 116, but located near the DCPV 114. Asdiscussed above, the inverter 106 is configured to convert the DC powerreceived from the DCPV 114 to an AC power suitable for delivery to theAC grid 102 at the grid frequency. It should be appreciated thatmultiple ACPVs 112 may be used to form a solar array with each ACPV 112having a dedicated inverter 106.

Referring now to FIG. 3, in one embodiment, the inverter 106 includes aninput converter 200 electrically coupled to a power bus 202, an outputconverter 204 electrically coupled to the power bus 202, and an activefilter 206 electrically coupled to the power bus 202. Additionally, insome embodiments, the inverter 106 includes a control circuit 208electrically coupled to the input converter 200, the output converter204, and the active filter 206 to control operations thereof. Asdiscussed in more detail below, the power bus 202 may be embodied as aDC bus or an AC bus. Accordingly, the input converter 200 may beembodied as a DC-to-DC converter or a DC-to-AC converter and the outputconverter may be embodied as a DC-to-AC converter or an AC-to-DCconverter.

In use, the inverter 106 is configured to be electrically coupled to theDC source 104 to receive a DC waveform therefrom. The inverter 106converts the DC waveform to a bus waveform, which may be a DC waveformor an AC waveform depending on the type of bus. Similarly, the outputconverter is configured to be electrically coupled to the AC grid 102and convert the bus waveform (i.e., either a DC waveform or an ACwaveform) to the output AC waveform at the grid frequency for deliveryto the AC grid 102.

As discussed above, the single-phase power output of the inverter 106includes an average component and a time-varying component due tovariations in the DC source 102 and/or demands of the AC grid 102. Thetime-varying component has a frequency substantially equal to twice theoutput AC waveform (i.e., the grid frequency). Without filtering, suchdouble-frequency power ripple must be supplied by the DC source 102(i.e., the double frequency ripple power propagates back and forthbetween the AC grid 102 and the DC source 102). Such demands on the DCsource 102 can result in failure or lower performance of the DC source102 and inverter 106. As such, the active filter 206 is used to reduceor substantially eliminate the double frequency power ripple occurringon the power bus 202 prior to the DC source 102. To do so, the activefilter 206 is configured to supply energy to and absorb energy from thepower bus 202 to thereby maintain a substantially constant bus voltageand reduce the amount of time-varying component required of the DCsource 104.

In typical inverters with passive filtering, the filter is placed on theinput side (i.e., on the primary side of the inverter transformer) tofilter the double-frequency power ripple on the low voltage bus from theDC source. Passive filters include a capacitive device, such as anelectrolytic capacitor, to perform the filtering functions. The amountof energy storable in a capacitor may be defined by:

W=0.5*C*V ²  Equation 2

wherein “W” is the amount of stored energy, “C” is capacitance, and “V”is the voltage.

As such, because the passive filter is coupled to a low voltage bus, alarger capacitance value is required to achieve a desired amount ofstored energy. Conversely, the active filter 206 is located on theoutput side (i.e., on the secondary side of the inverter transformer304—see FIG. 3) of the inverter 106 and coupled to the higher voltagepower bus 202. Because the power bus 202 has a relatively higher voltagethan the input side, a smaller value of capacitance may be used to storethe same amount of energy. Additionally, because the input bus is a lowvoltage bus, the change or ripple in the bus is relatively low. As such,the change in energy stored by the passive filter capacitor isrelatively low compared to the overall energy storage capability of thepassive filter capacitor. Further, the used energy storage of thepassive filter cannot be modified because there is no filter controlpresent. Conversely, by controlling the functions of the active filter206 via the control circuit 208, substantially all of the energy storagecapability of the active filter capacitor may be used (e.g., from nearly0 volts to the power bus 202 rail voltage). As such, because asignificantly greater amount of energy storage capability is used in theactive filter 206, a capacitor having a smaller capacitance value (and,thereby, a smaller footprint) may be used while still achieving thedesired amount of usable energy storage.

As well as reducing the physical size of the capacitor, the lowercapacitance requirement allows the use of alternative capacitortechnologies, which are typically lower cost and have increasedreliability, such as film capacitors. Additionally, placement of theactive filter on the output side of the inverter 106 may simplify thetransformer 304 of the inverter 106 in some embodiments (e.g., atwo-winding transformer may be used). As such, by placing the activefilter 206 on the output side of the inverter 106, the operatingefficiency of the inverter 106 may be increased because the lossesassociated with the double-frequency ripple power in the input converter200 are reduced or substantially eliminated.

As discussed above, the control circuit 208 is electrically coupled tothe input converter 200 and configured to control operation of the inputconverter 200 to convert the input DC waveform from the DC source 104 toa bus waveform at the power bus 202. In one particular embodiment, thecontrol circuit 208 may control the operation of the input converterbased on a maximum power point tracking (“MPPT”) algorithm ormethodology. For example, the control circuit 208 may include an MPPTcontrol circuit configured to execute an MPPT algorithm such as the MPPTalgorithm described in U.S. Patent Publication No. 2008/018338, entitled“Ripple Correlation Control Based on Limited Sampling” by Jonathan W.Kimball et al. To do so, the control circuit 208 may provide a pluralityof control signals to various circuits of the input converter 200 asdescribed in more detail below.

The control circuit 208 is also electrically coupled to the outputconverter 204 and configured to control operation of the outputconverter 204 to convert the bus waveform to the output AC waveformsuitable for delivery to the AC grid 102. In one particular embodiment,the control circuit 208 is configured to use a pulse width modulationalgorithm to control the output converter 204 such that the output ACwaveform is pulse width modulated. To do so, the control circuit 208 mayprovide a plurality of control signals to various circuits of the outputconverter 204 as described in more detail below.

Additionally, the control circuit 208 is electrically coupled to theactive filter 206 and configured to control the active filter to reducethe double-frequency power ripple on the power bus 202. In someembodiments, the active filter 206 is embodied as a current-controlledswitching converter. In such embodiments, the active filter 206 includesan inverter circuit and an energy storage device or circuit. In suchembodiments, the control circuit 208 is configured to control theinverter circuit of the active filter 206 to control a time-varyingcurrent of the energy storage circuit and generate an active filter ACwaveform at the energy storage device to thereby supply energy to orabsorb energy from the power bus 202. In some embodiments, the generatedactive filter AC waveform may have a frequency substantially equal to aharmonic of the grid frequency of the AC grid 102. For example, if thegrid frequency is 60 Hz, the active filter AC waveform may have afrequency of 60 Hz, 120 Hz, 180 Hz, etc. Additionally, in someembodiments, the active filter AC waveform may be shifted relative tothe AC grid frequency to improve the filtering function. For example, inone particular embodiment, the active filter AC waveform is shifted byπ/4 radians relative to a phase of the output AC waveform as describedin more detail in co-pending U.S. patent application Ser. No. 11/871,015entitled “Methods for Minimizing Double-Frequency Ripple Power inSingle-Phase Power Conditioners” by Philip T. Krein. et al.

Referring now to FIG. 4, in one embodiment the input converter 200 isembodied as a DC-to-DC converter. In such embodiments, the inputconverter 200 includes a boost converter 300, an inverter circuit 302, atransformer 304, and a rectifier 306. The boost converter 300 isembodied as an isolated boost converter and is electrically coupled tothe inverter circuit 302 and configured to be coupled to the DC source104. The inverter circuit 302 is embodied as a DC-to-AC inverter circuitconfigured to convert the DC waveform supplied by the DC source 104 toan AC waveform delivered to a primary of the transformer 304. Thetransformer 304 may be embodied two or more winding transformer having aprimary winding electrically coupled to the inverter circuit 302 and asecondary winding coupled to the rectifier 306. The transformer 304 isconfigured to convert the first AC waveform supplied by the invertercircuit 302 at the primary winding to second AC waveform at thesecondary winding. The first and second AC waveforms may havesubstantially equal frequency and may or may not have substantiallyequal voltages. The rectifier circuit is electrically coupled to thesecondary winding of the transformer 304 and configured to rectify thesecond AC waveform to a DC waveform supplied to the power bus 202.

One embodiment of a DC-to-DC input converter 350 is illustrated in FIG.5. The input converter 350 is electrically coupled to the DC source 104,embodied as a photovoltaic cell, via the boost converter 300. In theillustrative embodiment, the boost converter 300 is an isolated boostconverter embodied as an inductor 352. The input converter 350 alsoincludes a voltage claim 354 electrically coupled to the invertercircuit 302. The voltage clamp 354 may be embodied as a passive oractive circuit. For example, in embodiments wherein the voltage clamp354 is a passive circuit, a parallel diode and RC circuit may be used.The inverter circuit 302 is illustrative embodied as a bridge circuitformed by a plurality of switches 356, 358, 360, 362. Each of theswitches 356, 358, 360, 362 are configured to receive a correspondingcontrol signal, q_(IC1), q_(IC2), q_(IC3), q_(IC4), from the controlcircuit 208 to control operation of the inverter 302. The controlcircuit may use PWM to control the switches 356, 358, 360, 362 at arelatively high switching frequency (e.g., at a frequency that issubstantially higher than the AC grid frequency). As discussed above,inverter circuit 302 converts the DC waveform from the DC source 104 toa first AC waveform based on the control signals received from thecontrol circuit 208. In the illustrative embodiment, the invertercircuit 302 is a embodied as a full-bridge circuit, but other circuittopologies such as a half-bridge circuit may be used in otherembodiments. Additionally, although each of the switches 356, 358, 360,362 is illustrated as MOSFET devices, other types of switches may beused in other embodiments.

The illustrative transformer 304 includes a primary winding 364electrically coupled to the inverter circuit 302 and a secondary winding366 electrically coupled to the rectifier circuit 306. The transformer304 provides galvanic isolation between the primary side convertercircuitry (including DC source 104) and the secondary side circuitry(including power bus 202). The turns ratio of the transformer 304 mayalso provide voltage and current transformation between the first ACwaveform at the primary winding 364 and the second AC waveform at thesecondary winding 366.

The rectifier circuit 306 is electrically coupled to the secondarywinding 366 of the transformer 304 and configured to convert the secondAC waveform supplied by the transformer 304 to a DC bus waveformsupplied to the bus 202. In the illustrative embodiment, the rectifier306 is embodied as a full-bridge rectifier formed from a plurality ofdiodes 370, 372, 374, 376. Again, in other embodiments, other circuittopologies may be used in the rectifier circuit 306. The rectifiercircuit 306 may also include an energy storage device, such as acapacitor 378, for filtering the DC bus waveform.

Referring now to FIG. 6, in embodiments wherein the input converter 200is embodied as a DC-to-DC converter, the output converter 204 may beembodied as a DC-to-AC converter. In such embodiments, the outputconverter 204 includes an DC-to-AC inverter circuit 400 and anelectromagnetic interference (EMI) filter 402. The inverter circuit 400is electrically coupled to the power bus 202 and configured to convertthe DC bus waveform to the output AC waveform, which is filtered by theEMI filter 402. Additionally, the active filter 206 is electricallycoupled to the power bus 202 and configured to reduce thedouble-frequency power ripple of the power bus 202 as described above.

One embodiment of the inverter circuit 400 of the output converter 204and the active filter 206 is illustrated in FIG. 7. The active filter206 and the inverter circuit 400 are electrically coupled in parallel tothe power bus 200. The active filter 206 is embodied as a bridge circuit410 electrically coupled to an energy storage circuit 412. The energystorage circuit 412 is embodied as a series inductor 414 and capacitor416 circuit. The bridge circuit 410 is illustrative embodied as ahalf-bridge circuit formed from a plurality of switches 418, 420. Eachof the switches 418, 420 are configured to receive a correspondingcontrol signal, q_(AF1), q_(AF2), from the control circuit 208 tocontrol operation of the active filter 206. As discussed above, thecontrol circuit 208 may be configured to control the bridge circuit 410to control a time-varying current supplied to the energy storage circuit412 and generate an active filter AC waveform at the energy storagecircuit 412 having a frequency substantially equal to a harmonic thegrid frequency of the AC grid 102 to supply energy to and absorb energyfrom the power bus 202 and thereby reduce the double-frequency powerripple on the bus 202. In the illustrative embodiment, the bridgecircuit 420 is embodied as a half-bridge circuit, but other circuittopologies may be used in other embodiments. For example, as illustratedin FIG. 9, bridge circuit 410 may be embodied as a full-bridge circuitin some embodiments. In such embodiments, the bridge circuit 410 isformed from four switches 450, 452, 454, 456, each configured to receivea corresponding control signal, q_(AF1), q_(AF2), q_(AF3), q_(AF4), fromthe control circuit 208 to control operation of the active filter 206.The energy storage circuit 412 is located between the arms of thefull-bridge circuit 410 to form an H-bridge circuit. Additionally, othercircuit topologies may be used in other embodiments. Further, althougheach of the switches 418, 420, 450, 452, 454, 456 is illustrated asMOSFET devices, other types of switches may be used in otherembodiments.

As discussed above, the DC-to-AC inverter circuit 400 of the outputconverter 204 is electrically coupled to the power bus 202 in parallelwith the active filter 206. The inverter circuit 400 is configured toconvert the DC bus waveform to the output AC waveform for delivery tothe AC grid 102. The inverter circuit 400 is illustrative embodied as abridge circuit formed by a plurality of switches 430, 432, 434, 434.Each of the switches 430, 432, 434, 434 are configured to receive acorresponding control signal, q_(OC1), q_(OC2), q_(OC3), q_(OC4), fromthe control circuit 208 to control operation of the inverter 400. Asdiscussed above, the control circuit may use PWM to control the switches430, 432, 434, 434 to generate a pulse width modulated AC waveform.Again, it should be appreciated that although the illustrative theinverter circuit 302 is a embodied as a full-bridge circuit, othercircuit topologies such as a half-bridge circuit may be used in otherembodiments. Additionally, although each of the switches 430, 432, 434,434 is illustrated as MOSFET devices, other types of switches may beused in other embodiments.

On embodiment of the EMI filter 402 and other filtering circuitry isillustrated in FIG. 8. The EMI filter 402 is configured to filter theoutput voltage by reducing the conducted interference and satisfyingregulatory requirements. In the illustrative embodiment, the filter 402includes differential-mode inductors 440, 442, a line filter capacitor444, and common-mode inductors 446, 448.

Referring now to FIGS. 10 and 11, the transformer 304 of the inputconverter 200 may be embodied as a transformer 500 having a primarywinding 502, a secondary winging 504, and a tertiary winding 506. Insuch embodiments, the inverter circuit 302 is electrically coupled tothe primary winding 502 of the transformer 500 and the rectifyingcircuit 306 is electrically coupled to the secondary winding 504. Insuch embodiments, the inverter 106 includes an DC or AC bus in additionto the power bus 202. For example, as illustrated in FIG. 10, theinverter 106 includes a bi-directional rectifier 508, which is embodiedas a plurality of silicon rectifier diodes (SCRs), electrically coupledto the tertiary winding 506 and a DC bus 510. The bi-directionalrectifier 508 is configured to covert an AC waveform on at the tertiarywinding 506 to a DC bus waveform at the DC bus 510. In such embodiments,the bi-directional rectifier 508 also forms the bridge circuit 410 ofthe active amplifier 206, which controls a time-varying current of theenergy storage circuit 412 based on control signals received from thecontrol circuit 208 as discussed above. The active filter 206 iselectrically coupled to the DC bus 510 and configured to reduce thedouble-frequency power ripple of the power bus 202.

Alternatively, as illustrated in FIG. 11, the inverter 106 may includean AC power bus 512 coupled to the tertiary winding 506. In suchembodiments, the active filter 206 is electrically coupled to the AC bus512. However, the bridge circuit 410 is bi-directional and embodied aplurality of switches 520, 522, 524, 526. Illustratively, the switches520, 522, 524, 526 are embodied as MOSFETs, but other semiconductorswitching devices may be used in other embodiments to form thebi-directional bridge circuit 410. The switches are arranged in seriespairs (i.e., switches 520, 522 and switches 524, 526) in adrain-source-source-drain series connection. Again, in otherembodiments, other types of switches and circuit topology may be used.

Referring now to FIG. 12, in another embodiment, power bus 202 isembodied as an AC bus and the input converter 200 is embodied as aDC-to-AC inverter 600. In such embodiments, the input converter 600 mayinclude a boost converter 602, an inverter circuit 604, and atransformer 606. The boost converter 602 may be embodied as an isolatedboost converter and is electrically coupled to the inverter circuit 604and configured to be coupled to the DC source 104. The inverter circuit604 is embodied as a DC-to-AC inverter circuit configured to convert theDC waveform supplied by the DC source 104 to an AC waveform delivered toa primary of the transformer 606. The transformer 606 may be embodied asa two or more winding transformer having a primary winding electricallycoupled to the inverter circuit 604 and a secondary winding coupled tobus 202 (an AC bus in the present embodiment). The transformer 606 isconfigured to convert the first AC waveform supplied by the invertercircuit 604 at the primary winding to a second AC waveform at thesecondary winding. The first and second AC waveforms may havesubstantially equal frequency and may or may not have substantiallyequal voltages.

As illustrated in FIG. 13A-13D, the inverter circuit 604 may be embodiedas a voltage-sourced full bridge as shown in FIG. 13A, a current-sourcedfull bridge as shown in FIG. 13B, a voltage-sourced push-pullarrangement as shown in FIG. 13C, or current-sourced push-pullarrangement as shown in FIG. D. The full bridge inverters of FIGS. 13Aand 13B are formed from switches 610, 612, 614, 616 and the half-bridgeinverters of FIGS. 13C and 13D are formed from switches 618, 620. Theswitches 610, 612, 614, 616, 618, 620 are illustrative embodied asMOSFET devices, but may be embodied as power semiconductor devicescapable of being turned on and off with sufficiently high frequency.Furthermore, each power switch 610, 612, 614, 616, 618, 620 can bedriven with a gate-drive circuit. For the push-pull configurations ofFIGS. 13C and 13D, the power switches 618, 620 are switched on and offin alternating fashion, each with 50% duty cycle, in one embodiment. Forthe full-bridge configurations of FIGS. 13A and 13B, pairs of switches(marked q or 1−q) are switched on and off together, where q is either a0 (off) or 1 (on). In the push-pull configurations of FIGS. 13C and 13D,the transformer 606 includes a tapped primary winding 313.

In regard to the voltage-sourced configurations of FIGS. 13A and 13B,the alternating switching of the power switches 610, 612, 614, 616, 618,620 produces a square wave voltage across the primary windings of thetransformer 606 such that an approximately triangular flux linkagepattern is produced in the core of transformer 606. It should beappreciated that the flux in the magnetic core is approximately theintegral of the voltage applied to one of its coils. In the idealsituation, an average flux is zero, which occurs when a circuit isdriven perfectly symmetrically, and in the case of the push-pullconfigurations of FIGS. 13C and 13D, the primary windings of thetransformer 606 is tapped substantially in the middle.

In regard to the current-sourced configurations of FIGS. 13B and 13D, aseries inductor 622, 624, respectively, is used to provide asubstantially constant current to feed the inverter circuit 602. Thepower switches 610, 612, 614, 616, 618, 620 are alternated just as inthe voltage source configurations of FIGS. 13A and 13C to produce asquare wave current to the primary winding of the transformer 606.

In the push-pull arrangements of FIGS. 13C and 13D, the invertercircuits 604 include only two active switches 618, 620 coupled to groundsuch that high-side gate drives are not needed. However, because thecorresponding primary windings of the transformer 606 are tapped, thetransformer 606 is slightly more complex in these embodiments.Additionally, it should be appreciated that only half of the primarywinding is used at any given time. Further, because the power switches618, 620 are in direct series with the primary winding of thetransformer 606, a snubber circuit (not shown) may be used to prevent orotherwise limit over-voltages as the power switches 618, 620 are turnedoff.

In regard to the full-bridge arrangements of FIGS. 13A and 13B, thetransformer winding is not tapped and, as such, can be fully utilized. Asnubber circuit may or may not be used in these embodiments. However,additional power switches are required to form the full-bridge and ahigh-side gate drive is used.

Referring now to FIG. 14, in embodiments wherein the inverter 106includes the DC-to-AC input converter 600, the output converter 204 maybe embodied as an AC-to-AC output converter 700. In such embodiments,the output converter 700 includes an AC-to-AC frequency convertercircuit 702 and an electromagnetic interference (EMI) filter 704. Theconverter circuit 702 is electrically coupled to the (AC) power bus 202and configured to convert the AC bus waveform to the output AC waveform,which is filtered by the EMI filter 704. Additionally, the active filter206 is electrically coupled to the power bus 202 and configured toreduce the double-frequency power ripple of the power bus 202 asdescribed above.

Referring to FIGS. 15A and 15B, two embodiments of the converter circuit702 are shown. The converter circuit 702 may be embodied as afull-bridge arrangement 720 as illustrated in FIG. 15 A or a half-bridgearrangement 722 as illustrated in FIG. 16. Each embodiment includes anoutput filter 710 and an electromagnetic interference (EMI) filter 712.The full-bridge or half-bridge arrangements 720, 722 provide modulationof the output voltage of the transformer 606 such that, when passedthrough the output filter, a substantially smooth sine-wave voltage isproduced. The EMI filter 704 eliminates or reduces high-frequency noisefrom the output converter 702, as required for compliance to standardsand overall function of the inverter 106.

As shown in FIG. 15A, the full-bridge configuration 720 includes eightsilicon-controlled rectifiers (SCRs). Conversely, as shown in FIG. 15B,the half-bridge configuration 722 includes four SCRs. Additionally, thetransformer 606 of the full-bridge configuration 720 includes a singlesecondary winding. Conversely, the transformer 606 of the half-bridgeconfiguration 722 includes a tapped secondary winding. In should beappreciated that the full-bridge configuration 720, which comprises moreSCRs in series, has a simpler magnetic design compared to thehalf-bridge configuration 722, which has half as many SCRs and isrequired to block more voltage than the full-bridge configuration 720.

In either configuration, the SCRs are switched such that the outputwaveform from the secondary winding of the transformer 606 isalternately applied to the output filter 710. The timing of the SCRswitching may be based on a suitable PWM algorithm and controlled, forexample, by the control circuitry 208. The PWM control produces anoutput of the inverter 106 that is substantially sinusoidal. It shouldbe appreciated that each of the SCRs may include a gate circuit (notshown) to couple the on/off control signals from the control circuitry208 to the SCR gate terminals.

The output inductor 710 is used for coupling the AC grid 102 to theinverter 106. The output inductor 710 filters the output current suchthat limited harmonic distortion occurs in the output AC waveform.Additionally, the EMI filter prevents or minimizes unwanted noise fromcoupling to the AC grid 102.

Referring now to FIGS. 16 and 17, two additional illustrative systemtopologies are illustrated. In FIG. 16, the inverter 106 is embodied asa current-source inverter and includes half-bridges in the inputconverter 600 and the active filter 206 and a full-bridge in the outputconverter 700. Similarly, the inverter 106 is embodied as avoltage-source inverter in FIG. 17 and includes half-bridges in theinput converter 600 and the active filter 206 and a full-bridge in theoutput converter 700. It should be appreciated that the circuittopologies of FIGS. 16 and 17 are substantially similar. However, afiltering capacitor 800 is located at the output converter 700 in thecurrent-source inverter of FIG. 16 and located at the input converter600 in the voltage-source inverter of FIG. 17.

The inverter, controllers, and methods described herein may beimplemented as discrete circuits or in the form of software code and/orlogical instructions that are processed by a microprocessor, digitalprocessor, DSP or other means, or any combination thereof. The logicalprocesses may run concurrently or sequentially with respect to eachother or with respect to other processes, such as measurement processesand related calculations. Controllers may be implemented in mixed-signalcircuitry; in circuitry comprising mixed-signal circuitry comprising adigital processor core; or in circuitry comprising a combination ofmixed-signal circuitry and a separate digital signal processor. Suchcontrollers may be implemented as an integrated circuit or a hybriddevice. There may also be additional logical processes that may not beshown, such as, e.g., safety and protection mechanisms; timing andfrequency generation mechanisms; and hardware and processes related toregulatory requirements. Pre-determined values, such as, e.g., thecommanded values may be stored in read-only or re-programmablenon-volatile memory such as memory circuitry 110 or other storage media.Communication circuitry may also be incorporated into the inverter as ameans of downloading commanded values or other operating information tothe inverter and/or for uploading inverter operating information to userequipment.

Certain embodiments of the present disclosure have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the disclosure. Forexample, any of a wide variety of known non-resonant and resonantswitching power converter topologies may be used in place of thespecific converter embodiments described herein. The unipolar inputsource may be a fuel cell or another kind of DC source. The invertercontroller may comprise elements for regulatory and safety monitoringand control (e.g., circuits or processes for disabling the inverter inthe event of AC grid fault or input source fault; anti-islandingprotection). Switches in power converters (e.g., switches 171-174, FIG.2) are shown to be MOSFETs and to comprise diodes across theirterminals. It is understood that other types of switches may be used(e.g., bipolar transistors, IGBTs) and that diodes may be intrinsic tothe semiconductor switch or may be discrete devices. Switches may alsobe provided with passive or active snubbers to prevent losses and/or tolimit voltage or current stresses.

There is a plurality of advantages of the present disclosure arisingfrom the various features of the apparatuses, circuits, and methodsdescribed herein. It will be noted that alternative embodiments of theapparatuses, circuits, and methods of the present disclosure may notinclude all of the features described yet still benefit from at leastsome of the advantages of such features. Those of ordinary skill in theart may readily devise their own implementations of the apparatuses,circuits, and methods that incorporate one or more of the features ofthe present disclosure and fall within the spirit and scope of thepresent invention as defined by the appended claims.

1. An inverter for converting an input direct current (DC) waveform froma DC source to an output alternating current (AC) waveform for deliveryto an AC grid, the inverter comprising: an input converter electricallycoupled to a power bus and including a transformer, the input converterbeing configured to convert the input DC waveform to a bus waveformsupplied to the power bus; an output converter electrically coupled tothe power bus and configured to convert the bus waveform to the outputAC waveform at a grid frequency; and an active filter electricallycoupled to the power bus and configured to reduce a double-frequencyripple power of the bus waveform by supplying power to and absorbingpower from the power bus.
 2. The inverter of claim 1, wherein the powerbus is a DC bus and the bus waveform is a first DC waveform.
 3. Theinverter of claim 2, wherein the input converter comprises a firstinverter circuit electrically coupled to the DC source and a primarywinding of the transformer, the first inverter circuit being configuredto convert the input DC waveform to a first AC waveform at a firstfrequency and the transformer configured to converter the first ACwaveform to a second AC waveform at the first frequency.
 4. The inverterof claim 3, wherein the input converter further comprises a rectifiercircuit electrically coupled to a secondary winding of the transformerand the power bus, the rectifier circuit configured to rectify thesecond AC waveform to produce the first DC waveform.
 5. The inverter ofclaim 4, wherein the input converter comprises an isolated boostconverter electrically connected to the DC source and the first invertercircuit.
 6. The inverter of claim 5, wherein the active filter comprisesa bridge circuit electrically coupled to an inductor-capacitor (LC)circuit.
 7. The inverter of claim 4, wherein the output convertercomprises a second inverter circuit electrically coupled to the powerbus and configured to convert the first DC waveform to the output ACwaveform, and the active filter is configured to maintain asubstantially constant voltage across the second inverter circuit. 8.The inverter of claim 4, wherein the output converter comprises a secondinverter circuit electrically coupled to the power bus and configured toconvert the first DC waveform to the output AC waveform, and the outputAC waveform comprises an average component and a time-varying component,the active filter configured to supply substantially all of thetime-varying component of the output AC waveform.
 9. The inverter ofclaim 8, wherein the active filter comprises a third inverter circuitand an energy storage device.
 10. The inverter of claim 9, wherein thethird inverter circuit configured to control the time-varying componentof the output AC waveform by controlling a time-varying current of theenergy storage device.
 11. The inverter of claim 9, the third invertercircuit configured to generate a third AC waveform at the energy storagedevice, the third AC waveform having a frequency substantially equal toa harmonic frequency of the grid frequency.
 12. The inverter of claim 1,wherein transformer includes a tertiary winding, the active filter beingconnected to the tertiary winding.
 13. The inverter of claim 12, whereinthe active filter includes a rectifier circuit connected to the tertiarywinding, an inverter circuit connected to the rectifier circuit, and anenergy storage device.
 14. The inverter of claim 1, wherein thetransformer includes a tertiary winding and the power bus is a first ACbus, the inverter further comprising a second AC bus electricallycoupled to the tertiary winding.
 15. The inverter of claim 20, whereinthe active filter is connected to the second AC bus, the active filtercomprising an energy storage device connected to a center tap of thetertiary winding.
 16. An apparatus comprising: a solar panel comprisinga solar cell configured to generate a first direct current (DC) waveformin response to receiving an amount of sunlight; an inverter coupled tothe solar cell panel and configured to receive the first DC waveform andconvert the first DC waveform to an output alternating current (AC)waveform, the inverter comprising: an input converter electricallycoupled the solar cell and a DC bus, the input converter configured toconvert the first DC waveform to a second DC waveform supplied to the DCbus; an output converter electrically coupled to the DC bus andconfigured to convert the second DC waveform to the output AC waveformat a first frequency; and an active filter electrically coupled to theDC bus and configured to reduce a double-frequency ripple power of thesecond DC waveform by supplying power to and absorbing power from DCpower bus.
 17. The apparatus of claim 16, wherein the input convertercomprises a first inverter circuit and a transformer, the first invertercircuit (i) being electrically coupled to the solar cell and a primarywinding of the transformer and (ii) configured to convert the first DCwaveform to a first AC waveform at a first frequency, and thetransformer being configured to convert the first AC waveform to asecond AC waveform at the first frequency.
 18. The apparatus of claim17, wherein: the input converter further comprises a rectifier circuitelectrically coupled to a secondary winding of the transformer and theDC bus, the rectifier circuit configured to rectify the second ACwaveform to produce the second DC waveform, the output convertercomprises a second inverter circuit electrically coupled to the DC busand configured to convert the second DC waveform to the output ACwaveform, and the active filter comprises an energy storage device and athird inverter circuit configured to generate a third AC waveform at theenergy storage device, the third AC waveform having a frequencysubstantially equal to a harmonic frequency of the grid frequency. 19.An inverter comprising: a direct current (DC) bus, an input converterelectrically coupled to the DC bus, the input converter comprising (i) atransformer having a primary winding and a secondary winding, (ii) afirst inverter circuit electrically coupled to the primary winding andconfigured to convert an input DC waveform to a first alternatingcurrent (AC) waveform at the primary winding, the transform beingconfigured to convert the first AC waveform to a second AC waveform, and(iii) a rectifier circuit electrically coupled to the secondary windingand the DC bus, the rectifier circuit being configured to rectify thesecond AC waveform to produce a second DC waveform on the DC bus; anoutput converter electrically coupled to the DC bus, the outputconverter comprising a second inverter circuit electrically coupled tothe DC bus and configured to convert the second DC waveform to an outputAC waveform suitable for delivery to an AC grid; and an active filterelectrically coupled to the DC bus to reduce a double-frequency ripplepower of the second DC waveform by supplying power to and absorbingpower from DC power bus.
 20. The inverter of claim 19, wherein theactive filter comprises an energy storage device and a third invertercircuit configured to generate a third AC waveform at the energy storagedevice, the third AC waveform having a frequency substantially equal toa harmonic frequency of the grid frequency.